[css-ui-4] add the window-drag text (#13801)

* Adds the draggable CSS property

- adds `window-drag` to css-ui-4

* Adds window-drag

- changes the computed value
- updates line width styling

* Updates css-ui-4 with `window-drag`

Updates the spec text with the naming resolution by the CSSWG.

#13102

* Adding note about `app-region` alias

* Clarifying web event dispatch behavior and initial value

* Simplifying `window-drag` description

* Clarifying element's draggable target

* Minor tweaks

Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

---------

Co-authored-by: diekus <73939538+diekus@users.noreply.github.com>
Co-authored-by: Alexander Kyereboah <akyereboah@microsoft.com>
Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: 4 people

css-ui-4/Overview.bs

@@ -4153,6 +4153,59 @@ Single-Line Text Input Controls</h4>
 	* The content does not wrap
 	* The 'text-overflow' property applies regardless of the value of the 'overflow' property
 
+<h2 id="window-interaction">
+Window Interaction</h2>
+
+<h3 id="window-drag">
+Moving the window: the 'window-drag' property</h3>
+
+	<pre class=propdef>
+	Name: window-drag
+	Value: none | move
+	Initial: none
+	Applies to: all elements
+	Inherited: yes
+	Percentages: N/A
+	Computed value: as specified
+	Canonical order: per grammar
+	Animation type: discrete
+	</pre>
+
+	<wpt>
+	</wpt>
+
+	An installed application's window typically has a designated area where it can be dragged to move the window.
+	This area is generally at the top of the window
+	(usually part of the titlebar, toolbar, or both),
+	outside of the region that the page is actually rendered in.
+	For installed web apps, 
+	APIs or capabilities like [[WINDOW-CONTROLS-OVERLAY]]
+	can alter and remove the surfaces where users can move a window.
+	The 'window-drag' property allows the author to specify an area <em>within</em> the page
+	where a user can drag to move the window.
+	Values have the following meanings:
+
+	<dl dfn-for=window-drag dfn-type=value>
+		<dt><dfn>none</dfn>
+		<dd>
+			Specifies that the element cannot be used as a drag area.
+
+		<dt><dfn>move</dfn>
+		<dd>
+			Specifies that the element's [=principal box=] can be used as a [=window drag area=].
+	</dl>
+
+	Note: Some UAs may support <css>app-region</css> as a [=legacy shorthand=] for 'window-drag'.
+
+	When a [=box=] is a <dfn export>window drag area</dfn>,
+	a drag gesture on that element initiates a window move operation
+	instead of normal web event processing.
+	The UA must not fire pointer events, mouse events, or drag events
+	on the element for the duration of the window move gesture.
+
+	Note: This means the [[HTML]] <{html-global/draggable}> attribute
+	has no effect on elements with ''window-drag: move'';
+	the window move behavior takes precedence.
 
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